In biology, "membrane transport" refers to the transport of different substances through a biomembrane.
The interior of an area enclosed by a biomembrane (e.g. the cytoplasm of a cell) represents a biologically active, largely self-contained unit. This (selective and self-controlled) closure to the environment is necessary to allow the cell to build up its cellular environment, which is essential for its function. The biomembrane enables the cell to maintain the differences it has built up with respect to its environment. The double layer of the cell membrane, consisting of phospholipids, is only passable for gases and very small, mostly uncharged (and thus hydrophobic) molecules. It is not suitable for ions and most biologically active substances, since such molecules are polar and thus hydrophilic.
However, all life processes and all specific cell functions depend on the cell being able to be in contact with its environment and also communicating with it, e.g. through selective substance or particle exchange. In this respect, certain structures and mechanisms are necessary that selectively allow such molecules to pass through the membrane.